The present invention relates to a semiconductor manufacturing apparatus employed in a process necessary to heat a sapphire substrate in a process for manufacturing a semiconductor device by the sapphire substrate or a sapphire wafer using the same, and a method for manufacturing the semiconductor device.
A semiconductor device manufactured by a sapphire wafer in which silicon (Si) or the like has been epitaxially grown on a sapphire (Al2O3) substrate, is capable of greatly improving characteristics of a transistor by effects such as a reduction in parasitic capacitance, etc. as compared with a transistor of a semiconductor device manufactured by a silicon wafer using a silicon substrate.
Generally, a process for manufacturing a semiconductor device using a sapphire wafer can substantially follow a process for manufacturing a semiconductor device using a silicon wafer. A semiconductor manufacturing apparatus is shared and its manufacturing process can be produced at low cost.
Thus, when the manufacturing process using the silicon wafer is diverted to the manufacturing process using the sapphire wafer, a problem arises which results from the fact that the sapphire substrate is low in thermal conductivity as compared with the silicon substrate and thermal absorption efficiency is low due to non-absorption of light in a range from visible light to infrared light.
In the conventional semiconductor device, as to the low thermal absorption efficiency, an optical absorption body or a conductor is closely formed on the back surface of the sapphire wafer, and the sapphire substrate is radiant-heated or induction-heated by a lamp heating method or a high-frequency induction heating method to thereby efficiently heat the sapphire substrate (refer to, for example, a patent document 1 (Japanese Unexamined Patent Publication No. Hei 10(1998)-70313 (paragraph 0019 in page 4 to paragraph 0032 in page 5, FIG. 4 and FIG. 5)).
However, the above related art is accompanied by problems that in an atmospheric-pressure CVD apparatus employed in a manufacturing process in which since the sapphire substrate is heated from the back surface of the sapphire wafer, an atmosphere temperature in the corresponding process is low and the heating of the sapphire substrate is necessary, e.g., a process based on an atmospheric-pressure CVD (Chemical Vapor Deposition) method, a difference in temperature occurs between the front and back surfaces of the sapphire substrate due to the fact that the thermal conductivity of the sapphire substrate is low, warpage in the form of such a bowl that a peripheral edge portion of the sapphire wafer is lifted occurs in the sapphire wafer on a hot plate, and hence the apparatus encounters difficulties in sucking and holding the sapphire wafer by vacuum upon transfer or the like thereof.
In order to track down the cause of such warpage, the present inventors have carried out the following check tests.
FIG. 10 is an explanatory view showing the manner of warpage of a sapphire substrate.
In FIG. 10, reference numeral 1 indicates a sapphire substrate used as a substrate, which is a circular thin plate formed of a sapphire crystal.
Reference numeral 51 indicates a heating or hot plate which is heated by a heater 52. It is an experimental heating plate having the function of heating the sapphire substrate 1 placed on its upper surface.
Reference numeral 53 indicates a room temperature plate which is placed in a room and has the function of holding its atmosphere temperature, i.e., a temperature equivalent to the room temperature.
The atmosphere temperature is set to the room temperature and the heating plate 51 is heated by the heater 52 to set its temperature to 350° C. Thereafter, the warpage of the sapphire substrate 1 placed on the heating plate 51 was observed.
That is, the back surface of the flat sapphire substrate 1 is placed on the upper surface of the heating plate 51 (FIG. 10(a)). The sapphire substrate 1 begins to warp in bowl form from immediately after its placement. When 10 seconds or so have elapsed, only its central portion is brought into a state of being warped into bowl form in which it is placed in a state of contacting the upper surface of the heating plate 51 (FIG. 10(b)). This is because since the thermal conductivity of the sapphire substrate 1 is low, a temperature distribution occurs in the direction of thickness of the sapphire substrate 1, and the temperature of the heated back surface becomes high and the temperature of the front surface subjected to the atmosphere temperature becomes low, so that the difference in thermal expansion between the front and back surfaces of the sapphire substrate 1 occurs, whereby the sapphire substrate 1 is pulled to the front surface side small in elongation to thereby lift a peripheral edge portion of the sapphire substrate 1.
Thereafter, the warpage was held as it is even after the sapphire substrate 1 was left behind for 5 minutes or more (FIG. 10(c)). This is because only the central portion of the back surface of the sapphire substrate 1 placed in contact is thermally expanded by heating, and even though the peripheral edge portion is cooled, the sapphire substrate 1 cannot be restored to its original flat plate due to its thermal expansion.
When the sapphire substrate 1 heated in this way is placed on the upper surface of the room temperature plate 53 from its back surface while it is being held in the bowl form (FIG. 10(d)), the manner of its warpage is reversed at the moment of its placement, so that the sapphire substrate 1 is brought to warpage in the form of such a hanging bell that the central portion of the sapphire substrate 1 is raised (FIG. 10(e)). This is because since the central portion of the back surface of the convex sapphire substrate 1 is suddenly cooled by the room temperature plate 53 and thereby its temperature becomes lower than the temperature of the peripheral edge portion of the sapphire substrate 1, the central portion on the back surface side is shrunk and the elongation on the back surface side is reduced, so that the sapphire substrate 1 is pulled thereby to lift the central portion of the sapphire substrate 1.
Thereafter, when the sapphire substrate 1 is left as it is for a few minutes, the entire sapphire substrate 1 is cooled and brought to the room temperature, whereby the sapphire substrate 1 is restored to the original flat plate form.
Next, the heating plate set to 400° C. is provided at a position spaced away from the back surface of the sapphire substrate 1 by 5 mm, and the influence of heating under noncontact in an atmospheric state was confirmed.
FIG. 11 is a graph showing changes in temperature of the sapphire substrate heated in non-contact form.
In FIG. 11, the measurement of temperatures is performed using thermocouples embedded into the center of the front surface of the sapphire substrate 1 and its peripheral edge portion. A change in the temperature of the center with the elapse of time is indicated by a solid line and a change in the temperature of the peripheral edge portion is indicated by a broken line. For comparison, a change in the temperature of the center of a silicon substrate heated in like manner is indicated by a dashed line.
It is understood that while a rise in the temperature of the sapphire substrate 1 is slow as compared with the silicon substrate as shown in FIG. 11, a rise in the temperature of the center becomes fast from a point at which the temperature of the front surface of the sapphire substrate 1 exceeds the neighborhood of 200° C., and the temperature thereof suddenly rises at a point where the temperature thereof exceeds the neighborhood of 300° C., thereby reaching 380° C. The temperature of the peripheral edge portion of the sapphire substrate 1 is stable in the neighborhood of 300° C., and the change in temperature like the center of the sapphire substrate 1 does not appear in the change in the temperature of the center of the silicon substrate.
This shows that even in the case of heating based on the noncontact form, if heating is done from one surface alone, then heat is transferred to the surface (back surface in the above) on the side near the heating plate by heat transfer via air from the heating plate, so a difference in temperature occurs between the surface on the side near the heating plate and the surface (front surface) on the opposite side subjected to the room temperature, whereby warpage in the form of a bowl occurs in the sapphire substrate 1 in a manner similar to the case shown in FIG. 10(b), and its central portion approaches the heating plate so that a rise in temperature becomes fast, eventually resulting in contact of its central portion with the heating plate. When the sapphire substrate or the sapphire wafer is heated in contact or non-contact state from one surface in the process low in atmosphere temperature, similar warpage occurs even in the technique disclosed in the patent document 1.
Incidentally, the above warpage occurs in like manner even though the front and back surfaces are reversed. Even in the case of a sapphire wafer in which a device forming layer is formed in a sapphire substrate, similar warpage occurs.